MR AGENTS SENSITIVE TO BIOLOGICAL INDICATORS

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The major goals of this project are to develop applications of paramagnetic chemical exchange saturation transfer (PARACEST) imaging agents when bound to antibody surfaces and as small molecule sensors of hypoxia. MRI is the imaging modality of choice for soft tissue imaging but in general lacks sufficient sensitivity for molecular imaging of biological processes associated with cancer. Although Gd-based contrast agents are widely used in clinical MRI as non-specific extracellular agents, new approaches need to be developed to bring MRI into competition with optical and nuclear molecular imaging modalities. Paramagnetic complexes based on chemical exchange saturation transfer (PARACEST) offer a new mechanism for MRI contrast that could potentially improve sensitivity substantially and at the same time offer the unique ability to modulate imaging contrast (on/off) plus reflect specific tissue environments or physiology (pH, redox state, glucose levels). The first aim is to develop bifunctional ligands based on PARACEST for attachment to protein surface residues. These will be attached to model proteins and the water exchange characteristics of the resulting products will be evaluated and the lower detection limit of these systems evaluated by MRI. Two specific targeting systems will be evaluated, a phosphatidyl serine antibody and adenovirus particles with modified knob domain proteins, with the goal of creating activatible PARACEST systems that are completely "off" unless bound to their intended targets in vivo. A PARACEST agent will be developed that is trapped only in hypoxic tumor cells. The overriding goal of this project is to develop a new paradigm of molecular imaging agents for anatomical MR imaging of cancer based upon high sensitivity PARACEST agents.